Remove setting of Qt::AA_DontCreateNativeWidgetSiblings .

Change qtbase 72a7882cec07a9ad187c9e1772fb08f59a4b9519
avoids unnecessary creation of native siblings in Qt 5.

Task-number: QTBUG-34138
Change-Id: Ie6b847f57a40d56028c771628f31acb182179066
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>